Certain embodiments of the present invention are directed to integrated circuits. More particularly, some embodiments of the invention provide a system and method for intelligent control related to TRIAC dimmers by using modulation signal. Merely by way of example, some embodiments of the invention have been applied to driving light emitting diodes (LEDs). But it would be recognized that the invention has a much broader range of applicability.
A conventional lighting system may include or may not include a TRIAC dimmer that is a dimmer including a Triode for Alternating Current (TRIAC). For example, the TRIAC dimmer is either a leading-edge TRIAC dimmer or a trailing-edge TRIAC dimmer. Often, the leading-edge TRIAC dimmer and the trailing-edge TRIAC dimmer are configured to receive an alternating-current (AC) input voltage, process the AC input voltage by clipping part of the waveform of the AC input voltage, and generate an voltage that is then received by a rectifier (e.g., a full wave rectifying bridge) in order to generate a rectified output voltage.
FIG. 1 shows certain conventional timing diagrams for a leading-edge TRIAC dimmer and a trailing-edge TRIAC dimmer. The waveforms 110, 120, and 130 are merely examples. Each of the waveforms 110, 120, and 130 represents a rectified output voltage as a function of time that is generated by a rectifier. For the waveform 110, the rectifier receives an AC input voltage without any processing by a TRIAC dimmer. For the waveform 120, an AC input voltage is received by a leading-edge TRIAC dimmer, and the voltage generated by the leading-edge TRIAC dimmer is received by the rectifier, which then generates the rectified output voltage. For the waveform 130, an AC input voltage is received by a trailing-edge TRIAC dimmer, and the voltage generated by the trailing-edge TRIAC dimmer is received by the rectifier, which then generates the rectified output voltage.
As shown by the waveform 110, each cycle of the rectified output voltage has, for example, a phase angel (e.g., φ) that changes from 0° to 180° and then from 180° to 360°. As shown by the waveform 120, the leading-edge TRIAC dimmer usually processes the AC input voltage by clipping part of the waveform that corresponds to the phase angel starting at 0° or starting at 180°. As shown by the waveform 130, the trailing-edge TRIAC dimmer often processes the AC input voltage by clipping part of the waveform that corresponds to the phase angel ending at 180° or ending at 360°.
Various conventional technologies have been used to detect whether or not a TRIAC dimmer has been included in a lighting system, and if a TRIAC dimmer is detected to be included in the lighting system, whether the TRIAC dimmer is a leading-edge TRIAC dimmer or a trailing-edge TRIAC dimmer. In one conventional technology, a rectified output voltage generated by a rectifier is compared with a threshold voltage Vth_on in order to determine a turn-on time period Ton. If the turn-on time period Ton is equal to the duration of a half cycle of the AC input voltage, no TRIAC dimmer is determined to be included in the lighting system; if the turn-on time period Ton is smaller than the duration of a half cycle of the AC input voltage, a TRIAC dimmer is determined to be included in the lighting system. If a TRIAC dimmer is determined to be included in the lighting system, a turn-on voltage Von is compared with the threshold voltage Vth_on. If the turn-on voltage Von is larger than the threshold voltage Vth_on, the TRIAC dimmer is determined to be a leading-edge TRIAC dimmer; if the turn-on voltage Von is smaller than the threshold voltage Vth_on, the TRIAC dimmer is determined to be a trailing-edge TRIAC dimmer.
In another conventional technology, a rate of change of a rectified output voltage is used. The rectified output voltage is generated by a rectifier, and its rate of change is determined by quickly sampling the rectified voltage twice. Depending on the phase angles at which these two sampling actions are taken, a predetermined range for the rate of change is used. If the rate of change falls within this predetermined range, no TRIAC dimmer is determined to be included in the lighting system; if the rate of change falls outside this predetermined range, a TRIAC dimmer is determined to be included in the lighting system. If a TRIAC dimmer is determined to be included in the lighting system, whether the rate of change is positive or negative is used to determine the type of the TRIAC dimmer. If the rate of change is positive, the TRIAC dimmer is determined to be a leading-edge TRIAC dimmer; if the rate of change is negative, the TRIAC dimmer is determined to be a trailing-edge TRIAC dimmer.
If a conventional lighting system includes a TRIAC dimmer and light emitting diodes (LEDs), the light emitting diodes may flicker if the current that flows through the TRIAC dimmer falls below a holding current that is, for example, required by the TRIAC dimmer. As an example, if the current that flows through the TRIAC dimmer falls below the holding current, the TRIAC dimmer may turn on and off repeatedly, thus causing the LEDs to flicker. As another example, the various TRIAC dimmers made by different manufacturers have different holding currents ranging from 5 mA to 50 mA.
In order to solve this flickering problem, certain conventional technology uses a bleeder for the conventional lighting system. FIG. 2 is a simplified diagram of a conventional lighting system that includes a bleeder. As shown, the lighting system 200 includes a TRIAC dimmer 210, a rectifier 220, a bleeder 230, an LED driver 240, and LEDs 250. The TRIAC dimmer 210 receives an AC input voltage 214 (e.g., Vline) and generates a voltage 212. The voltage 212 is received by the rectifier 220 (e.g., a full wave rectifying bridge), which then generates a rectified output voltage 222 and a rectified output current 260. The rectified output current 260 is equal to the current that flows through the TRIAC dimmer 210, and is also equal to the sum of currents 232 and 242. The current 232 is received by the bleeder 230, and the current 242 is received by the LED driver 240. The magnitude of the current 232 may have a fixed magnitude or may change between two different predetermined magnitudes.
FIG. 3 is a simplified diagram showing certain conventional components of the bleeder as part of the lighting system 200 as shown in FIG. 2. The bleeder 230 includes a resistor 270 and a transistor 280. The transistor 280 receives a drive signal 282. If the drive signal 282 is at a logic high level, the transistor 280 is turned on, and if the drive signal 282 is at a logic low level, the transistor 280 is turned off.
For example, the TRIAC dimmer 210 is a trailing-edge TRIAC dimmer, the drive signal 282 remains at the logic low level, and the transistor 280 remains turned off. In another example, the TRIAC dimmer 210 is a leading-edge TRIAC dimmer as shown by a waveform 294, the drive signal 282 changes between the logic low level and the logic high level as shown by a waveform 292, and the transistor 280 is turned off and on.
As shown in FIG. 3, the waveform 290 represents the voltage 212 as a function of time for a leading-edge TRIAC dimmer as the TRIAC dimmer 210, and the waveform 292 represents the drive signal 282 as a function of time. If the rectified output current 260 becomes smaller than the holding current of the leading-edge TRIAC dimmer as the TRIAC dimmer 210, the drive signal 282 is generated at the logic high level in order to turn on the transistor 280 and increase the rectified output current 260.
FIG. 4 is a simplified diagram showing some conventional components of the bleeder as part of the lighting system 200 as shown in FIG. 2. The bleeder 230 includes a current detection circuit 310, a logic control circuit 320, and current sinks 330 and 340. As shown in FIG. 4, a current 350 is configured to follow through a resistor 360 in order to generate a voltage 370 (e.g., V1). The current 350 equals the rectified output current 260 in magnitude, and the voltage 370 represents the magnitude of the current 350. The voltage 370 is divided by resistors 362 and 364 to generate a voltage 372 (e.g., V2). The voltage 372 is received by the current detection circuit 310, which sends detected information to the logic control circuit 320. In response, the logic control circuit 320 either enables the current sink 330 with a control signal 332 or enables the current sink 340 with a control signal 342. The control signals 332 and 342 are generated by the logic control circuit 320 and are complementary to each other. If the current sink 330 is enabled, the current 232 received by the bleeder 230 is equal to a current 334; if the current sink 340 is enabled, the current 232 is equal to a current 344. The current 344 is larger than the current 334 in magnitude.
Returning to FIG. 2, the voltage 212 generated by the TRIAC dimmer 210 may have waveforms that are not symmetric between a positive half cycle and a negative half cycle of the AC input voltage 214. This lack of symmetry can cause the current that flows through the LEDs 250 to vary with time; therefore, the LEDs 250 can flicker at a fixed frequency (e.g., 50 Hz or 60 Hz). Also, the lighting system 200 often has only limited efficiency in energy consumption.
Hence it is highly desirable to improve the techniques of dimming control.